Dozens of major international clothing brands have expressed “grave concern” to Cambodia over the shooting deaths of five striking garment workers during a protest this month.

The brands including Puma, Wal-Mart, Gap, Levi and H&M have sent a joint letter to Prime Minister Hun Sen urging an investigation.

“The use of deadly force against protesting workers will not result in long-term industrial peace and jeopardises Cambodia’s position as a stable sourcing location for international brands,” it said.

“The investigation should ensure full accountability of any members of security forces found to have used disproportionate and excessive force.”

On January 3, five workers were killed and dozens injured in a clash with police during a protest demanding a higher minimum wage.

The brands also urged the government to respect the rights of 23 workers and union leaders detained since the protests, and to ensure the right to freedom of association.

“The global garment industry is changing rapidly, and industrial peace is required to rebuild our confidence in the Cambodian garment industry,” the letter said.

Garments are the country’s main export, with about 500 factories producing clothes and footwear for European and US labels. The sector employs more than 500,000 people, and was estimated to be worth about $US5 billion ($A5.72 billion) last year.